Glucose, sugarcane, starch, and celluloses are the most abundant renewable carbon sources found naturally on the earth. The high content of oxygenated functional groups in these carbohydrates has advantages in making use of them to produce fundamental chemicals. In particular, these carbohydrates are the most attractive feedstocks for intermediate chemical production in a sustainable way without emitting CO2.
Theoretically, two moles of lactic acid could be obtained from one mole of hexose either by fermentation or by catalytic reaction. Lactic acid itself is a monomer for the biodegradable polylactate synthesis. Lactic acid and its derivatives (such as alkyl lactates and polylactate) could act as platform compounds for the synthesis of other carbon-3 building blocks, such as propylene glycol, acrylic acid, and allyl alcohol for the productions of polymers.
Lactic acid is produced by the fermentation of glucose in present chemical industry. FIG. 1 shows the scheme for lactic acid and its derivatives preparation according to a commercial fermentation process. In the fermentation process, the concentration of lactic acid in the obtained water solution is very low. For example, the weight ratio of the lactic acid may be less than 10%. In addition, to isolate the lactic acid from the water solution, Ca(OH)2 should be added into the water solution, and Ca(OH)2 reacts with lactic acid thereby producing calcium lactate solid. Then, the calcium lactate solid is separated and added into H2SO4 solution. Accordingly, lactic acid is obtained, and CaSO4 solid precipitates in the lactic acid. Obviously, in the fermentation process described above, huge amounts of waste water and CaSO4 solid waste was produced, and only glucose can be used as the feedstock. Lactic acid could be produced from glucose in large scale (120,000 tons/year) in the existing fermentation processes. However, the biological processes generally suffer from low reaction rates and low product concentration (in water), resulting in long reaction times, larger reactors, and high energy consumption in the product purification process (Fermentation of Glucose to Lactic Acid Coupled with Reactive Extraction: Kailas L. Wasewar, Archis A. Yawalkar, Jacob A. Moulijn and Vishwas G. Pangarkar, Ind. Eng. Chem. Res. 2004, 43, 5969-5982).
It is known that, in the presence of aqueous alkali hydroxides, monosaccharides can be converted to lactic acid (R. Montgomery, Ind. Eng. Chem., 1953, 45, 1144; B. Y. Yang and R. Montgomery, Carbohydr. Res. 1996, 280, 47). However, the stoichiometric amount of base (Ca(OH)2) and acid (H2SO4) in the lactic acid recovery process would be consumed and, therefore, the stoichiometric amount of salt waste would be produced.
Although the commercial fermentation approach can produce large scale lactic acid, it only uses starch as a feedstock and the starch must be prehydrolyzed (or through fermentation) to glucose in advance. The fermentation process produces large amounts of waste water and solid waste (CaSO4). And the fermentation process for producing lactic acid includes many steps, which consume substantial amounts of energy. The infrastructure of the fermentation process is very complicated and uneconomical.